Easternarcher- Don't worry about building overkill, mine is 3/4" AC ply with the 1/4" thermax, but then again my kids get up on it and jump around. Also I have mine on a dimmer so I can regulate temp. easily.
Venom- Don't forget to drill some holes at each end to allow moisture to escape, and add a small fan. Keep a humidity meter in the box so you can keep an eye on that and temp. Too dry and you will have problems.